A gas turbine substantially comprises a compressor, a combustor, a turbine and an exhaust channel. In the compressor air is compressed in preparation for the combustion process. Fuel is injected into the combustor where it is mixed with the compressed air from the compressor and burnt. The hot and pressurized gas from the combustor is then directed towards the turbine blades to drive the turbine. After passing through the turbine, the combustion gas exits through the exhaust channel.
Document WO 02/12795 A2 discloses a combustor which operates with two different fuel flows. One of these fuel flows is referred to as pilot fuel flow. This fuel flow is injected directly into the combustion chamber and is the only fuel flow during the starting of the engine. Once the engine has reached a certain percent of its operating speed, premix fuel flow (main fuel flow) is added to the combustion chamber in addition to the pilot fuel. This premix fuel flow is mixed with air prior to entering the combustion chamber hence the name “premix”. By controlling the ratio of pilot flow to premix flow, the level of CO and NOx emissions can be adjusted. An example of a combustor with a pilot fuel flow and a premix fuel flow is disclosed in Sjunneson et al., International Publication No. WO 96/02796. The document WO 02/12795 A2 further discloses a control for adjusting the split between the premix and pilot fuel flows to control flame temperature and thus minimize emissions. A method of operating a combustor with two fuel flows is known. The operating method comprises the operating modes primary, lean-lean and premixed. Low NOx and CO emissions are only achieved in premix mode.
The pilot burner comprises injection and burning of pure fuel through primary nozzles, while the main burner comprises injection and burning of fuel premixed with air through secondary nozzles.
Rapid load reduction on certain gas turbine engines can result in flame failure due to pilot flow becoming too lean because of lags in the temperature measurements used to determine the required fuel split. For large load reductions, a minimum pilot split is triggered based on a step change in fuel demand, but this does not work on rapid ramped load changes where there is not a large enough step change in fuel flow.